1. Field of the Invention
Certain inventive aspects relate to the field of lithography. More particularly, they relate to methods and devices for characterizing, optimizing and performing lithography using electromagnetic radiation with short wavelengths, such as extreme ultraviolet lithography.
2. Description of the Related Technology
In the production of today's integrated circuits, optical lithography is one of the key techniques. The ongoing miniaturization of integrated circuits or other devices has resulted in the use of new types of lithography, e.g. in lithography using electromagnetic radiation with short wavelength. A typical example thereof is extreme ultraviolet lithography.
Whereas in conventional lithography techniques light typically is transmitted through the reticle, extreme ultraviolet lithography typically uses a reflective mask with a multi-layer coating. The illumination therefore typically is performed using an off-axis ring field illumination system that is non-telecentric with respect to the reticle side. In other words, illumination of the reticle typically is performed with a non-zero angle of incidence. By way of example, a schematic representation of a optical lithography system using a transmissive reticle is shown in FIG. 1a, while an optical lithography system using a reflective reticle is shown in FIG. 1b. FIG. 1a shows an illumination source 2, a transmissive reticle 4 and a device 6 comprising a resist layer 8. The resist layer 8 is illuminated with an illumination beam 10, modulated by the transmissive reticle 4, thus allowing to illuminate the resist in accordance with a predetermined pattern. For a system using a reflective reticle, as shown in FIG. 1b, an illumination source 22, typically an off-axis illumination system, generates an illumination beam guided to a reflective reticle 24. The illumination beam 30 is modulated by the reflective reticle 24 and reflected towards the substrate 6 comprising a resist layer 8.
In lithography applications using a transmissive reticle and lithography applications wherein the wavelength of the radiation used is substantially larger than the feature thickness on the reflective mask, typically a thin mask approximation (Kirchoff approximation) is valid. In lithography application wherein the wavelength of the radiation used is substantially of the same order of magnitude or smaller than the thickness of the reticle features 26, a thick mask approximation is to be used where light having a non-zero angle of incidence might be blocked by reticle features 26. In other words, the non-zero angle of incidence in combination with the three dimensional mask topography results in the so-called “shadowing effect”. The latter is illustrated in more detail in FIG. 1c, which is an enlarged view of part A of FIG. 1b. 
In Emerging Lithographic Technologies V, Proc. of SPIE 4343 (2001) 392-401, Krautschik et al. describe a comparison between a thin mask approximation and a thick mask approximation for extreme ultraviolet (EUV) lithography. It has been found that, using reflective EUV reticles, the critical dimension (CD) through focus behavior shows asymmetry and a shift in focus for thicker masks. The dependency on the angle of incidence, on the pitch, on the wavelength in the finite bandwidth of the illumination source and on the mask orientation has been studied.
In 21st Annual BACUS Symposium on Photomask Technology, Proc. of SPIE 4562 (2002) 279-287, Yan has studied the cause of the asymmetry of Bossung curves and best focus shifts for extreme ultraviolet lithography, i.e. where the thickness of the mask plays a role. It is discussed that these effects are due to a phase error at the reticle pattern edge. It furthermore has been shown that such effects may occur both for transmissive as for reflective masks. In U.S. Pat. No. 6,872,495, Schwarzl describes a method for fabricating a lithographic reflection mask for e.g. extreme ultraviolet lithography. Between the substrate and the reflection layer and/or on the side areas of the reflection layer, an absorber layer is applied allowing to reduce critical dimension changes due to shadowing of structures. The above described absorber layer allows projecting the patterned reflection layer in an undisturbed and image-faithful manner.
Furthermore, it is known that typically a number of aberrations are present in an optical system, such as a lithographic system. Spherical aberration may occur, which is the inability of the different zones of a lens/mirror to form an image all in one plane at the same distance of the lens/mirror. This typically results in the impossibility of making sharp focus. Coma may occur, which is a spherical aberration that passes light obliquely through the lens/mirror. This typically results in comet shaped images of a distant point, leading to blurred images as each detail is smeared. Astigmatism may occur, which is a non-uniform curvature of a refractive surface in an optical system resulting in blurred images as not all images are formed in the same point. Tilts in the Bossung curves, i.e. the CD behavior through focussing, also may be caused by these aberrations in the lithographic system or reticle.